“I don't think we need to be so hesitant in coming to the conclusion now that violence was the cause of death and not cholera, although these men might have had cholera in addition,” said University of Pennylvania anthropologist Janet Monge, who took part in the excavation.

The remains of the immigrants were uncovered in March 2009 by William E. Watson, a history professor at Immaculata University, and his twin brother Frank, a Lutheran minister. The brothers, who learned of the immigrants’ deaths in 2002, have been digging around the area since 2004. They have discovered thousands of artifacts, including pots, buttons and pipes.

Sevens sets of remains have been uncovered thus far, including four skulls. Each skull shows signs of physical trauma, and one appears to have a bullet hole. The Watsons feel that the discovery of the latest two skulls earlier this month serves as confirmation for their theory that the workers were murdered.

Duffy’s men were responsible for constructing Mile 59, described by Smithsonian magazine as “one of the toughest stretches” of the railroad. “The project required leveling a hill—known as making a cut—and using the soil to fill in a neighboring valley in order to flatten the ground,” it describes.

Within six weeks of the arrival, all 57 men were dead. They were buried in a mass grave near Malvern, Pa., and their deaths were kept a secret by the railroad company. Duffy ordered that the shanties were they lived be burned down for sanitary reasons.

The incident was part of local folklore, which was preserved by historian Julian Sachse, who interviewed elderly residents in the late 1800s. He was told that the railroad workers had attempted to flee the shantytown during a cholera outbreak, but the locals, fearing infection, would not let them inside their homes.

“The brothers were suspicious … as the disease's mortality indicates that only 40 to 60 percent of that group should have died,” writes the Philadelphia Inquirer. As they studied railroad documents, newspapers, diaries, and other sources, and dug around the area, they began to suspect that there may have been violence at play.

They “came up with a theory that local vigilantes, perhaps with the blessing of the contractor Phillip Duffy, simply came into the forest and killed all the workers, believing it was the only way to keep the cholera from spreading,” described NPR’s Guy Raz in a May 2010 interview with the brothers. “And right after the men were buried, Duffy had the site torched to hide the evidence.”

Immigrants were instrumental to the creation of railroads in the United States; Irish immigrants were some of the earliest workers. As the Library of Congress reveals, Irish workers often “took on the menial and dangerous jobs that were often avoided by other workers,” including railroad and canal construction. “Railroad construction was so dangerous that it was said, ‘[there was] an Irishman buried under every tie.’”

Cholera is “an acute, diarrheal illness caused by infection of the intestine with the bacterium Vibrio cholerae,” according to the Centers for Disease Control and Prevention. It still exists today in developing countries. In the United States, the disease was prevalent in the 1800s. It is spread through food and water, and “can spread rapidly in areas with inadequate treatment of sewage and drinking water.”